This is a revised application, the purpose of which is to test the hypothesis that the physiological requirements in adult humans for most of the nutritionally indispensable amino acids are considerably higher than suggested by current national and international recommendations. This hypothesis is supported on theoretical grounds and by initial data obtained from our recent series of investigations on the kinetics of metabolism of specific indispensable amino acids in adult humans receiving graded intakes of the test amino acid. We seek additional, definitive evidence to establish the validity, or otherwise, of this hypothesis. The specific aims are: 1. To include a tracer balance technique, in experiments designed to replicate, as closely as possible, the classical investigations on which currently accepted estimations of adult amino acid requirements largely have been based. 2. To validate further the concept of whole body tracer balance as a means of accurately quantifying the daily irreversible oxidation (loss) of specific indispensable amino acids. 3. To further explore the hypothesis that a reduced rate of body protein turnover occurs when amino acid intake is at current requirement levels, despite the possibility that overall body amino acid balance might be achieved at such low intakes. This will be accomplished by conducting relatively "long-term" metabolic nutritional studies, which include measures of lean body mass, immunocompetence and plasma proteins as indices of protein nutritional status, and the metabolic response to exercise. The long-term objective of these studies is, through and improved understanding of the characteristics and regulation of the integrative aspects of body nitrogen and amino acid metabolism, to develop more appropriate tools for the diagnosis of nutritional inadequacy, nutritional requirements and nutritional status. Our more immediate purpose is to try to define why the earlier and more classical estimates of amino acid requirements are so different from those based on more recent investigations of human amino acid kinetics.